This application discloses an invention that is related, generally and in various embodiments, to a compact three-phase multi-winding device.
For many years, three-phase transformers have generally been constructed with three coils of copper or aluminum windings which are installed on a laminated steel core having three legs. However, the conventional three-legged core design is not necessarily the optimal design in all cases. For example, due to the relatively large amount of space required for the conventional three-legged core design, the design is not necessarily the optimal design in many applications.
For applications where a conventional three-legged transformer requires a large number of three-phase secondary windings, each having specific voltages and phase angles, the secondary windings generally require two component windings from each coil (i.e., a total of six component windings from all three coils). For such applications, the secondary windings can be constructed, for example, in extended-delta, zig-zag, or polygon configurations. Since the number of turns in all components of any winding should be an integer, it is generally not possible to provide the specified voltage and/or phase angle exactly, and approximations are generally necessary. In the particular case when the secondary windings have the same nominal voltages, but different nominal phase angles with uniform spacing, the limited choices of integer turn numbers that can approximate the phase angles in the component windings may force secondary windings having different phase angles to have different voltages. Another problem with conventional three-legged transformers is the secondary windings will have different coefficients of coupling to the primary winding.
In general, the difficulty in physically implementing the approximations tends to increase the relative cost of the transformer, and the errors attending such approximations tend to degrade the harmonic cancellation within the transformer.
FIG. 1 illustrates an AC drive 1 which includes a conventional three-phase transformer 3. The transformer includes a primary winding 5 and a plurality of three-phase secondary windings 7, with each winding having specific voltages and phase angles. On the output side of AC drive 1, each of the three phases of the AC motor is driven by a string of power cells connected in series. In the AC drive of FIG. 1, there are six power cells per phase, labeled A1 through A6, B1 through B6, and C1 through C6, for a total of 18 power cells. It is appreciated that in other implementations, other numbers of cells per phase are possible (e.g., one cell, three cells, nine cells, etc.). In the context of an AC drive or an AC power supply, each power cell is a device which accepts three-phase AC input power, outputs a single-phase AC voltage, and includes an AC-DC rectifier (which may be regenerative), a smoothing filter, and an output DC-to-AC converter.
In the AC drive of FIG. 1, the transformer 3 receives three-phase AC input power from the local plant, at the points labeled R, S, and T on its primary winding 5. Each power cell receives three-phase AC input power from a dedicated secondary winding 7 of the transformer 3. The eighteen secondary windings 7 have the same nominal voltage, and are arranged in ranks of three, with each rank having one of six specific nominal phase angles. The nominal phase angles, which differ by multiples of ten degrees, are approximated using an extended-delta configuration and are referenced in FIG. 1 to the average phase angle of all the secondary windings. The different phase angles operate to cause harmonic cancellation within the transformer. Each secondary winding 7 contains six component windings, namely three inner delta component windings and three outer extension component windings. The component windings should all have integer numbers of turns, and the turn numbers or the inter-connections thereof are different for each phase angle.
The secondary winding component coils that are coupled to a given primary group of coils are vertically displaced around that primary coil with respect to each other. Because the various secondary component windings have different axial locations along the coils, they have different coupling coefficients to the primary winding 5. This tends to further degrade the harmonic cancellation in addition to the degradation caused by errors in voltage and phase-angle due to the requirement for integral turn numbers. Finally, as there is a relatively large amount of space which is unused in the construction of the conventional transformer 3, the overall size of the conventional transformer is relatively large.